In the microwave to millimeter wave (mm-wave) region, molecular spectroscopy is useful for constructing microwave or millimeter wave atomic clocks and other systems that require accurate timing signals. Atomic clocks excite the gas within a gas cell using a laser or mm-wave radiation induced by a RF transceiver, and the system detects the rotational quantum transition of dipolar molecules for a reference. The gas cell is ideally a hermetically sealed environment, including a desired target gas at a stable low pressure. By using advanced mass flow controllers and similar apparatus during sealing of the gas cell cavity, a moderate vacuum environment is practical to produce while simultaneously controlling a known amount of a volatile species. However, during cavity sealing operations, controlling the environmental conditions can be difficult and expensive.